Coaxial choke coupler



A. ALFORD COAXIAL CHOKE COUPLER May 7, 1963 2 Sheets-Sheet 1 OriginalFiled July 28, 1951 INVENTOR. Y Arm/raw Alford W W y 1963 A. ALFORD3,089,105

' COAXIAL CHOKE COUPLER Original Filed July 28, 1951 2 Sheets-Sheet 2FIG. 5

INVENTOR.

Andrew A I Ford BY 94 W%%% United States Patent 3,089,105 COAXIAL CHOKECOUPLER Andrew Alford, 299 Atlantic Ave., Winchester, Mass. Continuationof application Ser. No. 239,046, July 28, 1951. This application July10, 1956, Ser. No. 596,930 1 Claim. (Cl. 33373) The present inventionwhich is a continuation of my copending application Serial No. 239,046,filed July 28, 1951, now abandoned, relates to a coupler fortransmitting high, ultra high, or very high frequencies through astructure operated at a lower frequency and maintained above a groundpotential without introducing excessive reflections or radiation at thecoupler while at the same time not interfering or grounding thestructure or radiator operated at the lower frequency. More particularlythe device of the present invention may be called a coaxial chokecoupler, but the use of the words coaxia and choke are both taken in amore general sense than usually applied to coaxial lines or choke coils.The device of the present inventionmay have many uses, as for instancein mounting a high, ultra high, or super high frequency transmitter ontop of a low frequency insulated radio tower. In this case the problemis to transmit these very high frequencies through the tower which isinsulated and maintained at potential above the ground withoutdisturbing the existing electric system. In this case the so calledcoaxial choke coupler at the bottom of the tower will be designated topass the high frequencies, as for instance of an order from 100megacycles to 5000 megacycles and higher, so that the antenna on top ofthe tower may transmit such short waves. This problem also arises intransmitting both lower and high frequencies on planes. In such caseswhole sections of the stabilizer may act as an antenna for lowerfrequencies, such sections being insulated from the rest gof the plane.

Mounted on top of the stabilizer will be the high frequency antennawhich will be fed through a choke coupler between the insulated sectionof the stabilizer and the rest of the plane. The range of frequenciesusually defined for very high frequency, ultra high frequency and superhigh frequency, namely from 100 megacycles to 5000 megacycles and higherare readily transmitted through the coaxial choke coupler system of thepresent invention most advantageously. While the range is principallyapplied to frequencies over 100 megacycles, the same principle hereinset forth may be applied to lower frequencies within the allowablephysical dimensions of the choke coupler and the statement of rangeshould not apply as a limit of the scope of the invention.

The advantage of this arrangement of the present in' vention is that itintroduces a low value of capacitance and further that the radiationleakage is comparatively small. It is also possible by proper design tomaintain a substantially low standing wave ratio. Further the presentinvention may be readily adapted to any low frequency transmissionstructure for use with it of a very high frequency antenna within therange of frequencies covered by the present invention.

This invention further relates to and provides a so called choke couplerin which a frequency range is transmitted through a line in such a wayas to minimize radiation eifects at the coupling unit. In accomplishingthis, the impedance between successive sections of the inner and outerconductors are minimized, while at the same time substantial mismatchingis provided in the outer conductor joint so as to attenuate any highfrequency waves which may otherwise radiate from the joint and therebyinterfere with the low frequency transmission in the field in which thecoupler is located. In the several "ice modifications of the inventionthe outer conductor is pro vided with a so called disc line coupling, inwhich an improved coupling effect is obtained over those previouslyknown which utilize the sleeve coupling type of unit. In this disc typejoint there is less power radiation than from the sleeve type of joint.In one modification (see FIGURE 3), in addition to the disc lineelements set forth above, there is also provided a sleeve type of innpedance and a cavity type of impedance, thus waves radiating from thejoint of the outer conductor sections will successively meet a lowimpedance in the sleeve joint, a high impedance in the cavity, and a lowimpedance in the disc line, which impedance is lower than thatencountered in .the sleeve type of joint. These successive impedanceswhich are mismatched one with the other, when coupled together with thesignificant difference in characteristic impedance of air with the discline impedance, cooperate to effectively reduce the amount of powerradiation. This reduction of power radiation is quite noticeably largerthan that obtained with conventional and known joints as exemplified.

The invention will be more readily understood from the specification setforth below when taken in connection with the drawings illustrating anembodiment of the same, in which:

FIGURE 1 shows a section through .a coaxial choke coupler of the presentinvention.

FIGURE 2 shows a plan view of the arrangement of FIGURE 1.

FIGURE 3 shows a modification of the arrangement of FIGURE 1 in which athree step coaxial choke coupler is shown.

FIGURE 3a shows a top view of the modification of FIGURE 3, and

FIGURES 4 and 5 show application of the present invention to an antennatower and a stabilizer on a plane acting as an antenna.

In the arrangement shown in FIGURE 1, the coupling unit comprises anouter conductor 1 and an inner conductor 2. The outer conductor 1 isformed of two elements 3 and 4 which are insulated from each other in acentral section 5. The section 3 of the outer conductor is provided witha fiat outwardly extending flange 6 while the section 4 is also providedwith an outwardly extended flange 7 which flanges are positioned opposedto one another and separated by an insulator 8 which may also serve as adielectric. The two sections of the outer conductor are held together bymeans of an insulating ring 9 which fits over the shoulder of the flange7 of the section 4 and through which bolts or screws 10 are passedextending through the flange 6, capped by nuts 11 or other suitablemeans to clamp together the outer conductor sections about theinsulating element 8. The inner condoctor 2 is also formed of twosections 12 and 13 insulated from one another and separated by a space14 which may be filled with an insulator as indicated in FIGURE 1.Section 1'2 terminates in a small end 15 which extends into a recess inthe end of the conductor 13. The recess within the end 13 and theterminal end 15 of the conductor 12 may all be coaxial with one another,and the insulator filling the space 14 should also extend outwardly tocome between the shoulder 16 on the inner section 12 and the end of theinner section 13* so that sections 12 and 13 of the inner conductor areinsulated from one another.

The inner conductor is supported and centered co axially within theouter conductor by means of a central cylindrical tube 17 of insulatedmaterial upon either end of which rests insulators 18 and 19 which maybe in the shape of a frustrum of a cone. By such an arrangement exceptfor the impedance sections, the impedance along the line will be uniformthroughout. The outer conductor may be terminated at its ends byconductive fittings 20 and 21 which thread into the ends of sections 4and 3 respectively coaxially with the inner conductive ends 22 and 23respectively, which may be spaced and supported coaxially by cylindricalinsulators 24 and 25 respectively.

The choke coupler in this case provides a complete coupling unit withcoupling receptors at each end to which coaxial lines may fit, one goingup to the high frequency antenna and the other going to the power supplywhich may be on the ground side of the system.

In the choke coupler shown in FIGURE 1, two impedances are introduced inthe coupler, one which may be called Z, the impedance of the innerconductor in the section where the space 14 is situated which may befilled with an insulator as stated, and two, Z the plate or disc sectionin the outer conductor separated by the insulator 8. The effective wavelength of the section of the inner conductor along the insulating orspacing element 14, should be equal to approximately a physical quarterwave length at the center or mean frequency or some frequency within theband to be transmitted providing such length is substantially less thanone half the wave length of any frequency in the normal band. Even ifthe quarter wave length corresponded to a frequency well within thetransmission band, it would provide improved results, although thenearer to the mean frequency, the more effective will be theimprovement.

With regard to the so called disc impedance Z the reactance component ofthis impedance should be zero at the mean frequency and the disc line orsection is preferably designed to provide this result.

For this purpose the diameter of the plate sections is chosen atapproximately one-half wave length as corrected for the value of thedielectric serving as an insulafor.

I have found that the diameter should be approximately equal to maywhere K is the value of the dielectric constant. For 500 megacycles, forinstance, L 24. /2). would be equal to 12" and if some insulator is usedwhich has a dielectric constant of 4, the diameter of the disc should bein the neighborhood of 6''.

The effect on the propagation of waves through the coaxial line dependsupon the sum of the two impedances Z and Z When Z +Z is known, thestanding wave ratio introduced by the coaxial choke coupler can becalculated. In the present invention the choke coupler has a highreactive impedance at low frequencies and a low reactance at thefrequency of the high frequency source. By low reactance is meant, lowin comparison with 50 ohms in the range from 100: megacycles to 5000,

and by high impedance is meant an impedance of substantially 1000 ohmsor more at the low frequency transmission. A low impedance for the highfrequency will permit transmission at high frequencies withoutreflection and the construction of the choke coupler in accordance withthe present invention will effect this transmission through the chokecoupler without radiation.

The low impedance at high frequencies is in the outer '21s well as inthe inner conductor with the structure as set orth.

The arrangement described in FIGURES 1 and 2 is known as a single stagecoupler. In the sketch shown in FIGURE 3, a three stage coupler isshown. In this structure the power radiated through the gap between theplates 35 and 36 separated by the insulator must first pass through alow impedance line, then through a high impedance line, and finallythrough another low impedance line. When properly proportioned thisarrangement results in a very moderate low frequency capacitance and ahigh value of leakage index, that is a very small leakage.

The coupler constructed in accordance with FIGURE 3 may have a lowfrequency capacitance of 25 micromicrofarads and a leakage figure ofover 44 db. The good performance of the three stage coupler is due tothe fact that a three stage choke is effective even when the gapspacings described above are comparatively large.

The performance of the three stage coupler may be compared to that of asingle stage coupler of FIGURE 1, which with a gap of .032. had a lowfrequency capacitance C =42.=1 micro-microfarads and a leakage figurearound 30 db.

This capacitance C;- consists of two parts in the single stage coupler,the capacitance between the inner conductors C and the capacitancebetween the outer conductors C the total capacitance C being C plus C Inthe arrangement of FIGURE 3, the inner conductor 26 is made with a lowimpedance section 27 in which portions of the inner conductor 28 and 29are separated by the insulating element 30 similarly as described inconnection with FIGURE 1. This comprises a low impedance at highfrequencies and a high impedance at low frequencies. Between two coaxialconductive elements 31 and 32 there is provided a low impedance at highfre quencies and a high impedance is provided because of the element 31and the cavity 50 with the gap adjacent the lower section of the outerconductor, which impedance form a part of the coaxial line together withthe low capacitive plates in the outer conductor which will presently bementioned. The combination of these impedances provides over the highfrequency band of operation a very moderate low capacitance and a highvalue of leakage index. At no frequency in the operating range will thecombined impedance be a hindrance to efficient transmission. The sleeve3-2 within the sleeve 31 is provided with a shoulder 33' opposite theend of the sleeve 31. .The shoulder 33- is the end of a coaxial section33' which is joined to the coaxial outer conductor section 34 in thesolid section 34'. The section 33', 34 and the cavity 5 1 form a highimpedance which is substantially symmetrically positioned with the highimpedance just mentioned.

The outwardly and smaller inwardly extending flange 35 in the outerconductor forms part of the low frequency capacitance element with itsopopsing flange 36 connected to the bottom section of the outerconductor 37, as seen in FIGURE 3. v

The general construction of the elements follows along the same patternas described in connection with FIG- URES 1 and 2, with the exception ofthe additional impedance section formed by the coaxial elements 32 and31, 31 and 37, and 33' and 34. The sections 34 and 37 together with thetop section 38 as viewed in FIG- URE 3, complete the outer conductingcoaxial sleeve element which is coaxial with the inner conductor 26 tocomplete the three stage choke coupler.

FIGURE 4 shows the application of the invention to the installation ofvery high frequency antenna 39 on the top of a lower frequency radiotower 40. A coaxial cable 41 may carry transmitting current to theantenna 39 and this may be supplied through a coaxial choke coupler 42,one end of which is at ground potential and connect to the inputtransmission line 43. In this case the tower is insulated from theground by suitable insulators 44, 44.

FIGURE 5 shows the arrangement as applied to a very high frequencyantenna 45 mounted on the top of a stabilizer 46 of a plane 47. Thestabilizer 46 acts as a lower frequency radiator and the unit 45 has avery high frequency radiator. In this case the stabilizer 46 isinsulated from the rest of the plane by an insulator 48 and current issupplied to the very high frequency radiator 45 through a transmissionline 49 which has coaxial choke coupler 50 at the position of theinsulator 48 so that the stabilizer may be maintained above groundpotential for its low frequency transmission without interference.

As has been previously mentioned, the leakage figure in the arrangementof both the single stage and the three stage coupler is verysatisfactory and has been measured above 30 db in the case of the singlestage coupler and above 42 db in the three stage coupler.

At low radio frequencies, such as in the normal radio broadcast band andin the range generally called low radio frequency as compared with theshort wave bands, the choke coupler presents an impedance of 1000 ohmsor more, preferably 5000 ohms or more, while at the short wave bandwherein the coupler is used to transmit power the impedance is low incomparison to 50 ohms.

Consideration of the structure will show that ordinarily within theoperating band the reactance in the inner conductor of FIGURE 1, when itis approximately will be practically zero and slightly inductive whilethe reactance in the outer conductor will be low and slightly capacitivewithin the operating band.

What is meant by applicants previous reference to the diameter of theplate sections is that the common or rather coextensive sections of theplate should be approximately one-half wave length in diameter. Normallythe inner diameter of these plates is fairly small. This being the casethe overall width of the plates, that is the outer radius minus theinner radius, is substantially a quarter of a wave length in width. Aquarter of a wave length is desirable because such a width will presentan extremely low characteristic impedance at the inner edges of theserings or plates. The specific explanation of this phenomena involved anelaborate explanation utilizing Bessel functions. It may also be notedthat as this is a coupling device for transmission over a wide band,there is a fairly large leeway in the selection of the specific sizeutilized. This feature would explain the apparent divergence of thedrawings as shown in FIGURE 1 from this previously made explanation.

Having now described my invention, I claim:

A coaxial choke coupler for transmitting high frequency energy over aband in the range from megacycles to 5000 megacycles about a meanfrequency, said coupler comprising, coaxial inner and outer conductors,inner and outer insulating sections in said inner and outer conductorsrespectively, said sections dividing each conductor into first andsecond portions, said inner conductor insulating section beingcoextensive with the first and second portions of said inner conductoralong the axis common to said conductors for a distance substantiallyequal to a quarter wavelength for energy in said center conductorinsulating section of said mean frequency, said outer conductor having apair of opposed spaced flanges at adjacent ends of said first and secondportions thereof extending radially outward from and orthogonal to saidcommon axis, said outer conductor insulating section being a thinannular disk orthogonal to said common axis with opposite sides incontact with both opposed faces of said spaced flanges for a radialdistance substantially equal to a quarter wavelength for energy in saidouter conductor insulating section at said mean frequency, wherein saidouter conductor first portion includes an inner annular section coaxialabout said common axis formed with a sleeve end section extendingradially inward from a shoulder portion of said first inner annularsection, said outer conductor second portion includes an inner annularsection coaxial about said sleeve end section for nearly the entireaxial length of said sleeve end section, the end of said second portioninner annular section facing said shoulder portion across a gap which issubstantially bisected by the plane of said thin annular disk, saidouter conductor first and second portions also including an outerannular section from which said flanges extend, said first and secondouter conductor portions being coaxial about only one of said innerconductor portions.

References Cited in the file of this patent UNITED STATES PATENTS2,401,344 Espley June 4, 1946 2,451,876 Salisbury Oct. 19, 19482,465,922 Peterson Mar. 29, 1949 FOREIGN PATENTS 595,352 Great BritainDec. 3, 1947 598,375 Great Britain Feb. 17, 1948

